The present application relates to load power switching circuits and, more particularly, to a novel reverse-phase-control method and power switching circuit for controlling the conduction angle of devices applying A.C. current to a load.
It is well known to control the voltage and/or current applied to a load from an A.C. source by means of a phase-control circuit, in which a switching device is turn "on" to a conductive condition at a time after the zero crossing of the A.C. periodic waveform and the device, which is usually a regenerative switching device, is allowed to commutate "off" at the next zero crossing of the waveform. One of the more serious disadvantages of this type of phase-control circuit is that the load voltage and current undergo a sudden change when the switching device is turned "on" and this sharp change in current flow generates large amounts of electromagnetic interference (EMI). Another disadvantage of this type of phase-control circuit is the relatively high cost of such a circuit, when controlling loads having large in-rush current requirements. Thus, a tungsten filament lamp and the like loads, have relatively low "cold" resistances when first energized. The very large instantaneous current drawn by such a "cold" load requires that either the switching device be oversized to handle this large in-rush current, or that additional control subcircuits must be provided in the phase-control circuit, to slowly phase forward the conduction point an acceptable amount, until the load resistance has increased to the point that the load draws a "normal" operating current.
It is therefore highly desirable to provide a phase-control circuit which is not only capable of slowly increasing the device current, to provide an auto-soft-start capability for loads with high in-rush currents, but which also can provide reduced levels of EMI, with a relatively low circuit cost.